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I have been taking a few astro photos on a very high ISO, and have found what I think to be a few hot pixel. I though at first that I had managed to get a few very sharp pic of some red and blue stars. But now I have tried doing some stacking It seems that they are static to the rest of the moving sideways.

The thing that may have thrown me is that I thought that a 'hot pixel' was just that. A pixel, but it also seems to bleed outwards to make a small circle of a few faded pixels either side. Here is what I mean:

The image was saved as 'fine' quality on a D90 on a 2500mm telescope (1250mm with 2x teleconverter).

So are these hot pixels?

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The image you linked has expired. It is best to use the image tool to upload images to the local StackExchange content servers, so that temporal outages from other sites don't affect visibility here. –  jrista Mar 26 '11 at 1:30
    
Thanks :) I will do so next time! –  Designer023 Mar 28 '11 at 11:31
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1 Answer

up vote 9 down vote accepted

A hot pixel is actually one or more hot photo sensors. Most camera sensor chips are made up of red, green and blue photo sensors, usually placed in a pattern similar to this:

RGRGRGRGRG
GBGBGBGBGB
RGRGRGRGRG
GBGBGBGBGB

Each of these photo sensors ends up as a pixel in the final image, but as each photo sensor only has information about one color component, information is used from the surrounding sensors. That is why a single hot photo sensor bleeds into several pixels.

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Ah I see, so the one red pixel that is mis firing ends up having it's effect showing in the few around it on the image. Thanks :) –  Designer023 Mar 25 '11 at 10:26
    
yes, it causes a colour defect in multiple points in the RAW –  jwenting Mar 25 '11 at 14:46
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